Oversampling analog-to-digital converters generally consist of a modulator section which receives an analog signal and provides a serial data stream having a bit rate which is much greater than the Nyquist sampling frequency, followed by a digital filtering and decimation section which provides at its output a digitized representation of the analog input signal. Inside the modulator is an analog loop filter which is coupled to a summing node at its input and which provides an output that is digitized by a one bit analog-to-digital converter. The output of the one bit analog-to-digital converter forms the output of the modulator. The summing node sums the analog input signal with the output of the one bit analog-to-digital converter to provide an error signal which is input to the analog loop filter.
In the past, the analog loop filter has usually been comprised of either all single-ended integrators or all fully-differential integrators. Both the single-ended integrator and the fully-differential integrator have their advantages and disadvantages. For example, the single-ended integrator, while lacking the power supply rejection ratio (PSRR), linearity, and output swing of a fully-differential integrator, does enable an integration with only one feedback capacitor and input resistor as compared with the two capacitors and input resistors required of a fully-differential integrator. The fully-differential integrator, while requiring two sets of feedback capacitors which must be matched, does provide the increased PSRR, increased linearity (by minimizing the even order non-linearity in a discrete time integrator), and an increase in the differential output swing as compared to a single-ended output integrator.
Therefore it can be appreciated that an analog loop filter in an oversampling analog-to-digital modulator which is able to combine the favorable characteristics of a single-ended integrator and a fully-differential integrator while avoiding the major disadvantages of each type of integrator is highly desireable.